Recent advances in imaging techniques have revealed brown fat to be present in significant amount in adult humans. Using PET scanning, brown fat is shown to be more abundant in lean and younger individuals, suggesting that brown fat may regulate energy balance and metabolism. In a previous study we showed that following exposure to mild cold (19degree C/66degree F), the activity of brown fat was increased by nearly 10% and the energy expenditure by over 6%. Over time, these changes might result in substantial weight loss. However this has not been investigated and it is uncertain whether long-term cold exposure can sustain the increased activity of brown fat. In order to determine the therapeutic potential of brown fat activation, we designed a study investigating the effects of cold exposure over a long period.